Multilayer circuit board and method of manufacturing the same

ABSTRACT

A multilayer circuit board ( 1 ) includes resin bases ( 10   1  to  10   N ) stacked while placing separators ( 12   1  to  12   N−1 ) in between, interconnect patterns ( 11   1  to  11   N ) respectively formed on one surface of each of the resin bases ( 10   1  to  10   N ), and electro-conductive bumps ( 20   1  to  20   N−1 ) which electrically connect the interconnect patterns ( 11   1  to  11   N ). The resin bases ( 10   1  to  10   N ) and the separators ( 12   1  to  12   N−1 ) are heat-bonded, the separators ( 12   1  to  12   N−1 ) are composed of a first thermoplastic resin material having a first glass transition temperature, and the resin bases ( 10   1  to  10   N ) are composed of a second thermoplastic resin material having a second glass transition temperature higher than the first glass transition temperature.

TECHNICAL FIELD

The present invention relates to a multilayer circuit board, which is a stacked article of a plurality of resin bases, and a method of manufacturing the same, and in particular to a multilayer circuit board, which is a stacked article of a plurality of resin bases, each having an interconnect pattern which composes a passive element, and a method of manufacturing the same.

BACKGROUND ART

With recent trends towards higher density of integration of electronic instruments, a need has arisen for flexible circuit board having passive elements (inductor and coil antenna, for example) densely mounted thereon. Higher density of integration on the flexible circuit board may be embodied by stacking a plurality of resin bases, each having an interconnect pattern formed on one surface thereof, while placing an adhesive layer in between to thereby form a multilayer structure, typically as described in Patent Document 1 (Japanese Laid-Open Patent Publication No. H11-54934).

Prior art documents regarding the multilayer structure, besides Patent Document 1, may be exemplified by Patent Document 2 (Japanese Laid-Open Patent Publication No. 2008-103640) and Patent Document 3 (Japanese Laid-Open Patent Publication No. 2007-096121). The multilayer structure disclosed in Patent Document 2 has a structure in which circuit boards each having an interconnect pattern (electro-conductive pattern) and multilayer resin layers are alternately stacked, and each multilayer resin layer contains a first layer composed of a thermoplastic resin, a second layer composed of a thermosetting resin, and a connective conductor extended through the first layer and the second layer.

On the other hand, Patent Document 3 discloses a circuit board configured by an insulating base, having a conductor pattern formed on the surface thereof and composed of a thermoplastic resin, and an electro-conductive paste filled in via holes formed to extend through the insulating base. The multilayer structure disclosed in Patent Document 3 is configured by collectively or sequentially stacking a plurality of such circuit boards by heat bonding.

-   [Patent Document 1] Japanese Laid-Open Patent Publication No.     H11-54934 -   [Patent Document 2] Japanese Laid-Open Patent Publication No.     2008-103640 -   [Patent Document 3] Japanese Laid-Open Patent Publication No.     2007-096121

DISCLOSURE OF THE INVENTION

In order to form passive elements suitable for high-frequency devices into the multilayer structure, it is important to suppress energy loss ascribable to dielectric materials to a low level. Since the energy loss (dielectric loss) is proportional to the product of loss tangent (tan δ) and dielectric constant (∈), so that it is preferable that both of loss tangent and dielectric constant, and additionally moisture absorption ratio, are small. While the multilayer structure making use of the adhesive layers is disclosed in Patent Document 1 as described in the above, the multilayer structure is not understood as being suitable for high-frequency devices due to large loss tangent of the adhesive layers.

One possible method of embodying small loss tangent, small dielectric constant and small moisture absorption ratio relates to use of organic resin materials such as thermoplastic resin as materials for composing the multilayer structure. As disclosed in Patent Document 3, the multilayer structure may be manufactured by heat-bonding, and thereby integrating, a plurality of thermoplastic resin bases each having an interconnect pattern formed on the surface thereof. The passive elements (inductor, coil antenna and so forth) for high-frequency devices may be formed in the multilayer structure, by appropriately selecting geometry and combination of the interconnect patterns. The interconnect patterns formed on the thermoplastic resin bases may, however, deform due to melting or softening of the thermoplastic resin bases in the process of heat bonding. In this case, a large degree of deformation of the interconnect patterns may result in short-circuiting of the adjacent interconnect patterns, or undesirable formation of passive elements causative of malfunction.

Considering the above-described situation, it is therefore an object of the present invention to provide a multilayer circuit board which has expressing excellent electrical characteristics in high-frequency range, and includes interconnect patterns capable of ensuring a good geometrical accuracy.

According to the present invention, there is provided a multilayer circuit board which includes: a plurality of resin bases stacked while respectively placing a separator in between; a plurality of interconnect patterns formed on one surface of each of the plurality of resin bases; and electro-conductive bumps which are formed to extend through the resin bases and the separators, so as to electrically connect the plurality of interconnect patterns. The resin bases and the separators are heat-bonded. The separators are composed of a first thermoplastic resin material having a first glass transition temperature. Each of the resin bases is composed of a second thermoplastic resin material having a second glass transition temperature higher than the first glass transition temperature.

According to the present invention, there is also provided a method of manufacturing a multilayer circuit board, which includes: a step of forming, and thereby embedding, electro-conductive bumps respectively in (N−1) resin bases out of N resin bases (N is an integer of 2 or larger); a step of forming an interconnect pattern respectively on one surface of each of the N resin bases; a step of arranging the N resin bases by stacking them while respectively placing a separator in between, and while placing the resin base having no electro-conductive bumps, out of the N resin bases, outermostly; and a step of electrically connecting said plurality of interconnect patterns through said electro-conductive bumps, by heat-bonding, and thereby integrating, of said N resin bases and said separators after said N resin bases are stacked while placing said separators in between. The separator is composed of a first thermoplastic resin material having a first glass transition temperature. Each of the resin bases is composed of a second thermoplastic resin material having a second glass transition temperature higher than the first glass transition temperature. In the step of heat-bonding of the N resin bases and the separators, the N resin bases and the separators are heat-bonded at a temperature higher than the first glass transition temperature, and lower than the second glass transition temperature.

As described in the above, the multilayer circuit board of the present invention has a multilayer structure composed of a plurality of resin bases having the interconnect patterns formed on the surfaces thereof, and separators, which are heat-bonded with each other. Since the glass transition temperature of the thermoplastic resin, which is a material for composing the resin bases, is higher than the glass transition temperature of the thermoplastic resin, which is a material for composing the separators, so that the multilayer circuit board of the present invention is given a structure capable of preventing any geometrical deformation of the interconnect patterns on the resin bases, as a result of heat bonding between the resin bases and the separators proceeded at a temperature not causative of melting nor softening of the resin bases but contributive to melting or softening of the separators. Accordingly, the present invention may successfully provide a multilayer circuit board which has expressing excellent electrical characteristics (small dielectric constant and small dielectric loss) in high-frequency range, and includes the interconnect patterns capable of ensuring a good geometrical accuracy.

According to the method of manufacturing a multilayer circuit board of the present invention, since a plurality of resin bases having the interconnect patterns formed on the surfaces thereof and the separators may be heat-bonded at a temperature higher than the glass transition temperature of the separators (first glass transition temperature) and lower than the glass transition temperature of the resin bases (second glass transition temperature), so that the separators may be melted or softened without melting nor softening the resin bases. As a consequence, the present invention may successfully manufacture a multilayer circuit board which has expressing excellent electrical characteristics (small dielectric constant and small dielectric loss) in high-frequency range, and includes the interconnect patterns capable of ensuring a good geometrical accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings.

FIG. 1 is a drawing schematically illustrating a stacked structure of the multilayer circuit board according to a first embodiment of the present invention.

FIG. 2 is a drawing for explaining a method of manufacturing the multilayer circuit board of the first embodiment.

FIGS. 3(A) and (B) are plan views schematically illustrating exemplary interconnect patterns which compose a helical interconnect used for inductor or coil antenna.

FIG. 4 is a schematic drawing illustrating a helical interconnect.

FIG. 5 is a drawing schematically illustrating a stacked structure of the multilayer circuit board according to a second embodiment of the present invention.

FIG. 6 is a schematic drawing illustrating an exemplary interconnect pattern of the second embodiment.

FIG. 7 is a schematic drawing illustrating an exemplary interconnect pattern according to a modified example of the second embodiment.

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be explained below, referring to the attached drawings.

First Embodiments

FIG. 1 is a drawing schematically illustrating a stacked structure of a multilayer circuit board 1 according to the first embodiment of the present invention. As illustrated in FIG. 1, the multilayer circuit board 1 contains resin bases 10 ₁, . . . , 10 _(N) stacked while placing separators 12 ₁, . . . , 12 _(N−1) in between, interconnect patterns 11 ₁, . . . , 11 _(N) respectively formed on one surface of each of the resin bases 10 ₁, . . . , 10 _(N), and electro-conductive bumps 20 ₁, 21 ₂, . . . , 20 _(N−1) which electrically connect the interconnect patterns 11 ₁ to 11 _(N).

The electro-conductive bumps 20 ₁ to 20 _(N−1) are formed to extend through the resin bases 10 ₁ to 10 _(N−1) and the separators 12 ₁ to 12 _(N−1). In other words, the electro-conductive bump 20 _(k) (k represents any one of 1 to N−1) between every adjacent interconnect patterns 11 _(k), 11 _(k+1) is formed to protrude from one interconnect pattern 11 _(k) towards the other interconnect pattern 11 _(k+1).

Materials for composing the electro-conductive bumps 20 ₁ to 20 _(N−1) may be one or more species of metal materials selected from the group consisting of gold, silver, nickel, tin, lead, zinc, bismuth, antimony and copper.

High-frequency passive elements may be configured by the interconnect patterns 11 ₁ to 11 _(N) and the electro-conductive bumps 20 ₁ to 20 _(N−1). The high-frequency passive elements may be exemplified by resistor, inductor, capacitor, coil antenna, and combinations of these elements.

The separators 12 ₁ to 12 _(N−1) are composed of a thermoplastic resin material, and also the resin bases 10 ₁ to 10 _(N) are composed of a thermoplastic resin material. The thermoplastic resin material composing the resin bases 10 ₁ to 10 _(N) has glass transition temperature Tg2 which is higher than glass transition temperature Tg1 of the thermoplastic resin material composing the separators 12 ₁ to 12 _(N−1). By bonding the resin bases 10 ₁ to 10 _(N) and the separators 12 ₁ to 12 _(N−1) under heating in a temperature range higher than Tg1 and lower than Tg2, the multilayer circuit board 1 may be manufactured. In other words, by bonding, and thereby integrating the resin bases 10 ₁ to 10 _(N) and the separators 12 ₁ to 12 _(N−1) under heating at a temperature not causative of melting nor softening of the resin bases 10 ₁ to 10 _(N), but contributive to melting or softening of the separators 12 ₁ to 12 _(N−1), the interconnect patterns 11 ₁ to 11 _(N) on the resin bases 10 ₁ to 10 _(N) may be prevented from being geometrically deformed in the process of heat bonding.

The thermoplastic resin material composing the resin bases 10 ₁ to 10 _(N) and the thermoplastic resin material composing the separators 12 ₁ to 12 _(N−1) are respectively configured by cyclic olefinic resin compositions having different glass transition temperatures Tg2, Tg1, as their major constituents. The cyclic olefinic resin contains a (co)polymer of cyclic olefinic monomers as a major constituent. A desired glass transition temperatures Tg2, Tg1 may be obtained by varying polymerization conditions to thereby control molecular weight and density of crosslinkage of the cyclic olefinic resin. The glass transition temperatures Tg2, Tg1 may be elevated by increasing the degree of polymerization, or by elongating side chains of the cyclic olefinic resin.

In view of obtaining excellent electrical characteristics (small dissipation factor, small dielectric constant) in high-frequency range, norbornene resin is particularly preferably out of cyclic olefinic resin compositions. The norbornene resin may achieve electric characteristics represented by a dielectric constant of approximately 2, and a dissipation factor on the order of 10⁻³ to 10⁻⁴ at 10 GHz. Examples of the norbornene resin, having electrical characteristics represented by a dielectric constant of approximately 2, and a dissipation factor on the order of 10⁻⁴ at 10 GHz, include “TOPAS8007” (glass transition temperature=78° C.), “TOPAS6013” (glass transition temperature=138° C.), “TOPAS6015” (glass transition temperature=158° C.), “TOPAS5013” (glass transition temperature=134° C.), and “TOPAS6017” (glass transition temperature=178° C.), all of which are commercially available from Ticona.

In view of allowing the separators 12 ₁ to 12 _(N−1) to cause melting or softening in the process of heat bonding, without causing melting nor softening of the resin bases 10 ₁ to 10 _(N), difference of the glass transition temperatures (=Tg2−Tg1) between the resin base 10 ₁ to 10 _(N) and the separators 12 ₁ to 12 _(N−1) is preferably 50° C. or around. Combinations of the resin bases 10 ₁ to 10 _(N) and the separators 12 ₁ to 12 _(N−1) may be exemplified by “TOPAS5013” and “TOPAS8007”, while not being limited thereto.

Next, a preferable process of manufacturing the multilayer circuit board 1 will be explained.

First, a copper foil is formed on one surface of each of N resin bases 10 ₁ to 10 _(N). Next, electro-conductive bumps 20 ₁ to 20 _(N−1) are formed, and thereby embedded, in the resin bases 10 ₁ to 10 _(N−1) out of N resin bases 10 ₁ to 10 _(N). More specifically, through-holes which are bottomed on the copper foils are bored from the opposite surfaces of the resin bases 10 ₁ to 10 _(N−1). Then, electro-conductive projections are formed in the through-holes so as to project out therefrom, by electrolytic plating or paste printing. The electro-conductive projections may be configured by copper, for example. Highly accurate through-holes may readily be formed in the resin bases 10 ₁ to 10 _(N−1) by laser microfabrication. Next, the exposed surfaces of the electro-conductive projections are covered with a metal or alloy. The metal may be composed of at least one species selected from the group consisting of gold, silver, nickel, tin, lead, zinc, bismuth and antimony, and may be composed of a single layer, or two or more layers. The alloy may be exemplified by solders composed of at least two or more species of metals selected from the group consisting of tin, lead, silver, zinc, bismuth, antimony and copper. The solders may be exemplified by those of tin-lead-base, tin-silver-base, tin-zinc-base, tin-bismuth-base, tin-antimony-base, tin-silver-bismuth-base, and tin-copper-base, while being not limited thereto, and allowing selection of most suitable one. Of course, the metals and alloys enumerated in the above have melting points sufficiently higher than the glass transition temperatures of the resin bases 10 ₁ to 10 _(N) or the separators 12 ₁ to 12 _(N−1), and do not melt in the process of heat bonding.

Next, the interconnect patterns 11 ₁ to 11 _(N) are formed by etching the copper foils formed on N resin bases 10 ₁ to 10 _(N). The interconnect patterns 11 ₁ to 11 _(N) are typically such as those preliminarily designed to configure high-frequency passive elements such as inductor, resistor, capacitor or matching circuit. At least a part of the interconnect patterns 11 ₁ to 11 _(N) formed in this process may preferably be a spiral interconnect pattern having an angle of turn exceeding 360°, detail of which will be described later.

Next, as illustrated in FIG. 2, the resin bases 10 ₁, . . . , 10 _(N) are stacked while placing the separators 12 ₁, . . . , 12 _(N−1) in between, in a molding machine. In this process, the resin bases 10 ₁ to 10 _(N) are arranged so as to direct all of the interconnect patterns 11 ₁ to 11 _(N) in the same direction, while placing the resin base 10 _(N), having no electro-conductive bump formed thereon, outermostly.

Next, the resin bases 10 ₁ to 10 _(N) and the separators 12 ₁ to 12 _(N−1), disposed in the molding machine, are pressed under heating in a temperature range higher than the glass transition temperature Tg1 of the separators 12 ₁ to 12 _(N−1), and lower than the glass transition temperature Tg2 of the resin bases 10 ₁ to 10 _(N). By the heating, the resin bases 10 ₁ to 10 _(N) and the separators 12 ₁ to 12 _(N−1) are heat-bonded. The electro-conductive bumps 20 ₁, . . . , 20 _(N−1) extend through the melted or softened separators 12 ₁, . . . , 12 _(N−1), and are bonded to the surfaces of the interconnect patterns 11 ₂, . . . , 11 _(N).

FIG. 3(A) and FIG. 3(B) are plan views illustrating the interconnect patterns 11 _(k), 11 _(m) which compose a helical interconnect used for inductor or coil antenna. The helical interconnect may be configured by a combination of the interconnect pattern 11 _(k) illustrated in FIG. 3(A) and the interconnect pattern 11 _(m) illustrated in FIG. 3(B). The interconnect pattern 11 _(k) illustrated in FIG. 3(A) is a spiral interconnect having bump connection points 21 _(k), 22 _(k) at the ends thereof, and has an angle of turn of approximately 360°. The interconnect pattern 11 _(m) illustrated in FIG. 3(B) is a spiral interconnect having bump connection point 21 _(m), 22 _(m) at the ends thereof, and has an angle of turn exceeding 360°. While each of the interconnect patterns 11 _(k), 11 _(m) illustrated in FIG. 3 is given in a form of combination of curve(s) and a straight line, they may be configured by curve(s) only, or by straight line(s) only.

Note that “helical” herein means a geometry such as rotatively ascending or descending in the thickness-wise direction of the multilayer circuit board 1. On the other hand, “spiral” herein means a geometry such as turning while departing from, or approaching, the center axis in the plane of the multilayer circuit board 1.

The helical interconnect may be formed by connecting the bump connection point 22 _(k) formed at one end of the interconnect pattern 11 _(k) illustrated in FIG. 3(A), with the bump connection point 22 _(m) formed at one end of the interconnect pattern 11 _(m) illustrated in FIG. 3(B), through the electro-conductive bump. Alternatively, the helical interconnect may be formed by connecting the bump connection point 21 _(k) formed at the other end of the interconnect pattern 11 _(k) illustrated in FIG. 3(A), with the bump connection point 21 _(m) formed at the other end of the interconnect pattern 11 _(m) illustrated in FIG. 3(B), through the electro-conductive bump.

FIG. 4 is a schematic drawing illustrating a helical interconnect formed by using the interconnect patterns 11 _(k), 11 _(m) illustrated in FIGS. 3(A), (B). The interconnect patterns 11 ₁, 11 ₃, . . . , 11 _(N−1) (where, N represents an even number) in the odd-th layers have a geometry same as that of the interconnect pattern 11 _(k) illustrated in FIG. 3(A), whereas the interconnect patterns 11 ₂, 11 ₄, . . . , 11 _(N) in the even-th layers have a geometry same as that of the interconnect pattern 11 _(m) illustrated in FIG. 3(B). The interconnect patterns 11 ₁ to 11 _(N) are electrically connected while respectively placing inter-layer connecting lines 23 ₁ to 23 _(N−1) configured by the electro-conductive bumps 20 ₁ to 20 _(N−1) in between.

Effects expressed by the multilayer circuit board 1 and the method of manufacturing the same according to the first embodiment are as follow.

In the process of manufacturing the multilayer circuit board 1, the resin bases 10 ₁ to 10 _(N) having the interconnect patterns 11 ₁ to 11 _(N) formed on the surfaces thereof are heat-bonded with the separators 12 ₁ to 12 _(N−1), at a temperature higher than the glass transition temperature Tg1 of the separators 12 ₁ to 12 _(N−1), and lower than the glass transition temperature Tg2 of the resin base 10 ₁ to 10 _(N), so that the thermoplastic resin composing the separator 12 ₁ to 12 _(N−1) may be melted or softened without melting nor softening the thermoplastic resin composing the resin bases 10 ₁ to 10 _(N). Since the separators 12 ₁ to 12 _(N−1) and the resin bases 10 ₁ to 10 _(N) are pressed under heating under such conditions, so that the electro-conductive bumps 20 ₁, . . . , 20 _(N−1), which are formed on the resin bases 10 ₁ to 10 _(N), kept unmelted and unsoftened, may be bonded to the surfaces of the interconnect patterns 11 ₂, . . . , 11 _(N), after being extended through the melted or softened separators 12 ₁, . . . , 12 _(N−1). In this way, geometrical modification of the interconnect patterns 11 ₁ to 11 _(N) in the process of heat bonding may be avoidable.

Accordingly, the present invention may successfully provide the multilayer circuit board 1 which has expressing excellent electrical characteristics (small dielectric constant and small dielectric loss) in high-frequency range, and includes the interconnect patterns 11 ₁ to 11 _(N) capable of ensuring a good geometrical accuracy.

The interconnect patterns 11 ₂, 11 ₄, . . . , 11 _(N) in the even-th layers have an angle of turn exceeding 360°, while giving an almost circular geometry in portions thereof opposed to the interconnect patterns 11 ₁, 11 ₃, . . . , 11 _(N−1) in the odd-th layers configured on the upper and lower sides thereof. If the angle of turn of the interconnect patterns 11 ₁ to 11 _(N) were smaller than 360°, the number of turns would be reduced corresponding to the angle of turn which comes short of 360°, and thereby inductance of the helical interconnect would decrease. In other words, the helical interconnect in this embodiment may be formed without degrading the inductance.

Second Embodiment

Next, the second embodiment of the present invention will be explained. FIG. 5 is a drawing schematically illustrating a stacked structure of a multilayer circuit board 2 of the second embodiment. FIG. 6 is a schematic drawing illustrating exemplary interconnect patterns 11 ₁ to 11 ₁₇ which compose a helical interconnect formed in the multilayer circuit board 2. The multilayer circuit board 2 of the second embodiment has a configuration same as that of the multilayer circuit board 1 of the first embodiment, except for positions of the electro-conductive bumps 20 ₁ to 20 ₁₆ and geometry of the interconnect patterns 11 ₁ to 11 ₁₇, and may be manufactured by processes same as those for the multilayer circuit board 1 of the first embodiment.

Each of the interconnect patterns 11 ₁ to 11 ₁₇ is a spiral interconnect pattern having an angle of turn exceeding 360°. The interconnect patterns 11 ₁, 11 ₃, . . . , 11 ₁₇ formed on the resin bases 10 ₁, 10 ₃, . . . , 10 ₁₇ in the odd-th layers, and the interconnect patterns 11 ₂, 11 ₄, . . . , 11 ₁₆ formed on the resin bases 10 ₂, 10 ₄, . . . , 10 ₁₆ in the even-th layers have geometries turned over from the other. In other words, since the resin bases 10 ₁, 10 ₃, . . . , 10 ₁₇ and the resin bases 10 ₂, 10 ₄, . . . , 10 ₁₆ are alternately disposed, the interconnect patterns 11 ₁ to 11 ₁₇ are alternately stacked face-up and face-down. Of course, the interconnect patterns 11 ₁ to 11 ₁₇ are connected at both ends thereof through the electro-conductive bumps.

In the first embodiment, as illustrated in FIG. 1 and FIG. 4, the electro-conductive bumps 20 ₁, 20 ₃, . . . , 20 _(N−1) (where, N represents an even number) in the odd-th layers were formed at positions which overlap with each other when viewed in the direction of stacking, and also the electro-conductive bumps 20 ₂, 20 ₄, . . . , 20 _(N−2) in the even-th layers were formed at positions which overlap with each other when viewed in the direction of stacking. As a consequence, the multilayer structure of the first embodiment may result in non-uniform thickness, depending on types of the thermoplastic resin adopted or conditions of heating or pressing.

In contrast, according to the second embodiment, since the interconnect patterns 11 ₁ to 11 ₁₇ are alternately stacked face-up and face-down, so that positions of formation of the electro-conductive bumps 20 ₁ to 20 ₁₆ do not overlap when viewed in the direction of stacking. Since there are sixteen resin bases 10 ₁ to 10 ₁₆ having the electro-conductive bumps 20 ₁ to 20 ₁₆ embedded therein, so that the inter-layer connecting lines 23 ₁ to 23 ₁₆ may be arranged at regular angular intervals around the center axis of the interconnect patterns 11 ₁ to 11 ₁₇, by forming the interconnect patterns 11 ₁ to 11 ₁₇ while adjusting the angle of turn thereof to (360°+360°/16). More specifically, as illustrated in FIG. 6, the interconnect patterns 11 ₁ to 11 ₁₇ are electrically connected through the inter-layer connecting lines 23 ₁ to 23 ₁₆ formed by the electro-conductive bumps 20 ₁ to 20 ₁₆. The inter-layer connecting lines 23 ₁ to 23 ₁₆ are arranged at an angular interval of approximately 22.5° around the center axis of the interconnect pattern 11 ₁ to 11 ₁₇.

As explained in the above, since the electro-conductive bumps 20 ₁ to 20 ₁₆ of the multilayer circuit board 2 of the second embodiment are formed at positions not overlapped when viewed in the direction of stacking, so that the non-uniformity in the thickness may be avoidable. Accordingly, the multilayer circuit board 2 may be improved in the rigidity balance, and consequently in the strength thereof.

While the number of the layers in the multilayer circuit board 2 of the second embodiment was seventeen, the number is not limited thereto, so that the multilayer circuit boards 2 may be modified to have eighteen or more layers. For an exemplary case where the number of the layers in the multilayer circuit board 2 is N, the number of the resin bases 10 having the electro-conductive bumps 20 embedded therein is N−1, so that the inter-layer connecting lines 23 may be arranged at regular angular intervals around the center axis of the interconnect patterns 11, by forming the interconnect patterns 11 while adjusting the angle of turn thereof to (360°+360°/N−1).

One modified example of the second embodiment may possibly be given by a multilayer circuit board having a helical interconnect as illustrated in FIG. 7. As is clear from FIG. 7, the odd-th inter-layer connecting lines 23 ₁, 23 ₃, 23 ₅, 23 ₇ are formed so as to be arranged at an angular interval of approximately 45° around the center axis of the interconnect patterns 11 ₁ to 11 ₈, and the even-th inter-layer connecting lines 23 ₂, 23 ₄, 23 ₆ are formed so as to be arranged at regular angular intervals around the center axis of the interconnect patterns 11 ₁ to 11 ₈.

The embodiments of the present invention were described in the above referring to the attached drawings merely for exemplary purposes, while allowing adoption of various configurations other than those described in the above. For example, while the first and second embodiments dealt with the cases having six or more layers of interconnect patterns, the present invention is not limited thereto. An embodiment of the multilayer circuit board having at least two layers of the interconnect patterns may possibly be given as a modified example of the first embodiment, and an embodiment of the multilayer circuit board having at least two layers of the interconnect patterns may possibly be given as a modified example of the second embodiment.

An inductor may be embodied by a plurality of spiral interconnect patterns connected in a helical manner as illustrated in FIG. 4 and FIG. 6, where the geometry of the interconnect patterns is not limited thereto. The geometry may alternatively be arc-like, square-like or polygonal.

Moreover, the multilayer circuit board of the present invention may be provided with layers other than those composing the above-described interconnect patterns, resin bases and separators, without contradicting the features of the present invention.

This application claims priority right based on Japanese Patent Application No. 2008-279732 filed on Oct. 30, 2008, the entire content of which is incorporated hereinto by reference. 

1. A multilayer circuit board comprising: a plurality of resin bases stacked while respectively placing a separator in between; a plurality of interconnect patterns formed on one surface of each of said plurality of resin bases; and electro-conductive bumps which are formed to extend through said resin bases and said separators, so as to electrically connect said plurality of interconnect patterns, said resin bases and said separators being heat-bonded, said separators being composed of a first thermoplastic resin material having a first glass transition temperature, and each of said resin bases being composed of a second thermoplastic resin material having a second glass transition temperature higher than said first glass transition temperature.
 2. The multilayer circuit board as claimed in claim 1, wherein each of said electro-conductive bumps is formed to protrude from one of adjacent interconnect patterns of said plurality of interconnect patterns, towards the other.
 3. The multilayer circuit board as claimed in claim 1, wherein said first and second thermoplastic resin materials are respectively configured by cyclic olefinic resin compositions having different glass transition temperatures, as their major constituents.
 4. The multilayer circuit board as claimed in claim 3, wherein said cyclic olefinic resin composition is a norbornene resin.
 5. The multilayer circuit board as claimed in claim 1, wherein said electro-conductive bumps are composed of one or more species of metal materials selected from the group consisting of gold, silver, nickel, tin, lead, zinc, bismuth, antimony and copper.
 6. The multilayer circuit board as claimed in claim 1, wherein said plurality of interconnect patterns and said electro-conductive bumps configure a passive element.
 7. The multilayer circuit board as claimed in claim 6, wherein said passive element contains any one of, or a plurality of, circuit(s) selected from resistor, inductor and capacitor.
 8. The multilayer circuit board as claimed in claim 6, wherein said plurality of interconnect patterns are connected to form a helix as a whole, through said electro-conductive bumps.
 9. The multilayer circuit board as claimed in claim 8, wherein said interconnect patterns are respectively formed on three or more resin bases, and said electro-conductive bumps are formed at different positions when viewed in the direction of stacking.
 10. The multilayer circuit board as claimed in claim 9, wherein at least a part of said plurality of interconnect patterns is a spiral interconnect pattern having an angle of turn exceeding 360°, and one end of said spiral interconnect pattern connects to one interconnect pattern through one electro-conductive bump, and the other end connects to other interconnect pattern through other electro-conductive bump.
 11. The multilayer circuit board as claimed in claim 10, wherein said plurality of spiral interconnect patterns are stacked alternately upside-down, with the individual ends thereof connected to each other through said electro-conductive bumps.
 12. The multilayer circuit board as claimed in claim 11, wherein the number of said resin bases having said electro-conductive bumps buried therein is M (M is a natural number), and the angle of turn of said spiral interconnect pattern is (360°+360°/M).
 13. A method of manufacturing a multilayer circuit board, comprising: forming, and thereby embedding, electro-conductive bumps respectively in (N−1) resin bases out of N resin bases (N is an integer of 2 or larger); forming an interconnect pattern respectively on one surface of each of said N resin bases; arranging said N resin bases by stacking them while respectively placing a separator in between, and while placing the resin base having no electro-conductive bumps, out of said N resin bases, outermostly; and electrically connecting said plurality of interconnect patterns through said electro-conductive bumps, by heat-bonding, and thereby integrating, of said N resin bases and said separators after said N resin bases are stacked while placing said separators in between, said separator being composed of a first thermoplastic resin material having a first glass transition temperature, each of said resin bases being composed of a second thermoplastic resin material having a second glass transition temperature higher than said first glass transition temperature, and in said heat-bonding of said N resin bases and said separators, said N resin bases and said separators are heat-bonded at a temperature higher than said first glass transition temperature, and lower than said second glass transition temperature.
 14. The method of manufacturing a multilayer circuit board as claimed in claim 13, wherein in said forming, and thereby embedding, said electro-conductive bumps, said electro-conductive bumps are formed to protrude from one surface towards the other surface of each of said N−1 resin bases.
 15. The method of manufacturing a multilayer circuit board as claimed in claim 13, wherein said first and second thermoplastic resin materials are respectively configured by cyclic olefinic resin compositions having glass transition temperatures different from each other.
 16. The method of manufacturing a multilayer circuit board as claimed in claim 15, wherein said cyclic olefinic resin compositions are norbornene resins.
 17. The method of manufacturing a multilayer circuit board as claimed in claim 13, wherein said electro-conductive bumps are composed of one or more species of metal materials selected from the group consisting of gold, silver, nickel, tin, lead, zinc, bismuth, antimony and copper.
 18. The method of manufacturing a multilayer circuit board as claimed in claim 13, wherein, in said forming said interconnect patterns, a spiral interconnect pattern having an angle of turn of 360° or larger is formed on at least a part of said N resin bases, and in said heat-bonding of said N resin bases and said separators, one end of said spiral interconnect pattern is connected through one electro-conductive bump to one interconnect pattern, and the other end is connected through other electro-conductive bump to other interconnect pattern.
 19. The method of manufacturing a multilayer circuit board as claimed in claim 18, wherein, in said forming said interconnect patterns, said spiral interconnect pattern is formed on said first resin base, and an interconnect pattern equivalent to the turned-over of said spiral interconnect pattern is formed on the second resin base, in said arranging said resin bases, said first resin base and said second resin base are alternately arranged, and in said heat-bonding of said N resin bases and said separators, one end of said interconnect pattern on said first resin base is connected to one end of interconnect pattern on said second resin base, through said electro-conductive bump.
 20. The method of manufacturing a multilayer circuit board as claimed in claim 19, wherein in said forming said interconnect patterns, each of said spiral interconnect patterns is formed to have an angle of turn of (360°+360°/(N−1)). 